1. Field of the Invention
The present invention relates to a method of generating a threshold matrix which is compared with a grayscale original image in halftoning the original image, a technique for generating image data with use of the threshold matrix which is generated by the method, an image recording apparatus using the threshold matrix generated by the method, and also relates to a recording medium in which data of the threshold matrix generated by the method is recorded.
2. Description of the Background Art
Conventionally, an AM (Amplitude Modulated) screening where gray levels are represented by changing the size of dots which are regularly arranged (accurately, the dots are clusters each of which is a group of dots connecting one another), has been used in creating a halftone image for printing from an original image of gray scale (i.e., continuous tone). An FM (Frequency Modulated) screening where gray levels are represented by changing the number of dots of certain size, the dots being irregularly arranged, has been used for the original image in which moire easily arises. The FM screening is specifically used in a printer with a relatively low recording resolution of dots (e.g., an inkjet printer) in many cases.
In actually halftoning the original image, a threshold matrix where a plurality of elements are arranged in a row direction and a column direction and a threshold value is assigned to each of the elements is generated in advance and prepared, and the threshold matrix is compared with the original image. In a technique disclosed in Japanese Patent Gazette No. 2622429 (Document 1) which is a technique for generating the threshold matrix, an image is generated by applying a blue noise filter to a binary dot profile representing an arrangement of dots in a gray level and a position where a dot should be added in the dot profile is specified on the basis of a gray level of each pixel in a differential image between the image and the original dot profile, to generate a dot profile in the next gray level of the above gray level. This process is repeated to acquire a dot profile in each gray level and thereby, a blue noise mask (threshold matrix) is generated.
In an inkjet printer which performs printing by moving a head in a predetermined scan direction, the head having a plurality of outlets arranged in a width direction perpendicular to the scan direction, there is a case where streak unevenness (hereinafter, referred to as “longitudinal streak”) extending in the scan direction appears in a printed halftone image (printed image) because of variations in ejection directions of inks from the plurality of outlets, or the like. When a halftone image is generated with use of the threshold matrix with isotropy in Document 1, the longitudinal streak easily appears in a middle grayscale range in the halftone image. On the other hand, Japanese Patent Application Laid-Open No. 2006-14271 (Document 2) discloses a technique for suppressing appearance of longitudinal streak or graininess in a color printed image printed with use of a threshold matrix, where threshold matrixes of color components are generated so that growing of halftone dot areas with increase of gray level in one color component in the original image shows different directionality (directivity) from another color component. The scan direction in a printer normally corresponds to the row direction or the column direction in a threshold matrix.
In the threshold matrix generated by the technique of Document 2, however, since the halftone dot areas grow (i.e., clusters become larger) from a plurality of base points, which are randomly arranged, with increase of gray level, the density of clusters is inevitably decreased (becomes coarse) to decrease the reproduction in the halftone image of thin lines, characters and the like in the original image. For this reason, required is a novel threshold matrix (especially, in a printer with a low resolution) which is capable of reducing graininess in a halftone image with directionality in a direction tilted relatively to (directions corresponding to) the row and column directions, and further improving the reproduction of thin lines, characters and the like.